This invention relates to sorting apparatus. It is particularly concerned with sorting apparatus which grades particles in a flowing stream according to their color characteristics, and activates an ejection mechanism based on that grading to remove unacceptable particles from the stream.
A particular color sorting apparatus of the above type is available from Sortex Limited of London, England under the designation Sortex 5000. That apparatus uses a bichromatic system for scanning particulate material in free flow through air, which system grades each particle in the stream, and instructs ejectors located downstream to remove from the stream particles not matching the predetermined acceptance criteria.
Various sorting apparatus which grade particulate material according to its ability to reflect light in different wavelength ranges are described in U.S. Pat. Nos. 4,203,522; 4,513,868; and 4,699,273, the disclosures whereof are incorporated herein by reference. In apparatus disclosed in the '522 patent detectors are responsive to light reflected from the particles in different wavelength ranges and generate signals indicative of different qualities of the product. These signals are compared and analyzed, to generate a comparison signal which can activate an ejector to remove the relevant particle from the product stream.
Problems can arise in sorting apparatus of the above general type if some individual particles in the product stream are of different sizes. A larger dark product can in some circumstances reflect more total light than a much smaller light object. These problems can to some extent be met by the use of carefully selected background colors, but this solution usually involves a degree of compromise, even where a line scan system is employed. One of the problems in a line scan system is that spaces between products can appear as for example, dark defects. To obtain a matched background across the whole extent of the line scan the variation in illumination across the corresponding particles would have to be correlated both in color and brightness to the background. Even if this were attainable, it would be difficult to maintain in operation. A further degree of enhancement and flexibility in bichromatic sorting may be achieved by creating a say, red/green Cartesian map divided into accept and reject portions. Any background would limit and complicate the full implication of such a method of operation. Thus, the best solution is to eliminate the background from the color measurement.